1. FIELD OF THE INVENTION
This invention relates to a valve for controlling both the flow of steam from a steam header and air into a steam box of a web drying machine. More particularly, this invention relates to a control valve for the steam box of a paper drying machine for controlling the moisture profile of a paper web in the cross machine direction.
2. INFORMATION DISCLOSURE STATEMENT
In the papermaking art, paper or board is produced by supplying, under pressure, a jet of pulp or stock onto a moving screen. The pulp usually includes over 95% by weight, of water, and such water is removed by drainage through the moving forming screen. The resultant paper web is removed from the forming screen and is fed through a plurality of pressing rolls for removing excess quantities of water from the formed web. The pressed web is transferred from the press section to a drying section where the relatively moist web is fed around a plurality of rotating drying cylinders.
Due to variations of pressure applied by the press rolls in the cross machine direction during the pressing stage, and due to forming and drying variables, the web of paper entering the drying section will vary in moisture content along the width, or cross machine direction, of the moving web. Such variations in moisture content, or web profile, unless compensated for, will result in a paper web which will exhibit variable coating characteristics along the cross machine direction. Furthermore, by controlling the cross machine direction web moisture content, a much higher percentage of moisture can be allowed in the salable roll of paper. Also, optimum control of the cross machine direction web dryness will minimize the number of sheet breaks.
Consequently, much research has been carried out in order to compensate for such cross machine direction profile in order to provide a paper web having a relatively uniform moisture content along the width thereof. The steam box is disposed adjacent a rotating web guide roll which may be a suction roll or a suction box with the web passing around the web guide roll and disposed between the roll and the steam box. The steam box is supplied with steam from a steam header with the steam header extending longitudinally through the steam box. The steam box is divided into a plurality of equally-sized compartments which are disposed in side-by-side relationship across the width of the web such that steam can be individually and selectively supplied to one or more of the compartments. Steam within a compartment is diffused onto the adjacent portion of the moving web in order to effect a drying operation thereto. Monitoring means are disposed downstream of the steam box in order to ascertain the particular moisture content of the various portions of the width of the web. ln accordance with control signals supplied by such monitoring means, steam is selectively supplied to one or more of the steam box compartments to compensate for any irregularity in the moisture profile of the moving web.
In copending application, Ser. No. 720,482 to Chance et al and assigned to Beloit Corporation, rather than merely providing a steam control valve for controlling the flow of steam from the header into the steam box, it has been found advantageous that when such flow of steam is to be reduced or terminated, a corresponding volume of air be admitted into the steam box. Such admission of compensating air has been found useful in not only increasing the sensitivity of the moisture profiling apparatus, but also has improved the smooth flow of steam into, or out of, the steam box.
Copending application, Ser. No. 720,482 to Chance et al and assigned to Beloit Corporation, is incorporated by reference into the present application.
In the aforementioned proposals of pending application, No. 720,482, a flow of air has necessitated the provision of a second valve with separate control means. Such second air control valve and control means have posed relatively complex control problems resulting in a relatively high cost of manufacture.
The present invention overcomes the inadequacies of the aforementioned devices and provides a control valve having a significant advantage over these earlier proposals, thereby contributing to a significant improvement in the art of paper web moisture profiling.
An object of the present invention is the provision of a control valve for moisture profiling having a valve stem with a first and second valve closure which is activated by a stepping motor such that when the valve stem is moved, the change in flow of steam into the steam box is inversely proportional to the change in flow of air into the steam box.
Another object of the present invention is the provision of a control valve in which, as the first valve is opened to admit steam into the steam box, the second valve for controlling the flow of air is proportionately closed to reduce the flow of air into the steam box compartment.
Another object of the present invention is the provision of a control valve in which, as the first valve is closed thereby cutting off the supply of steam to the steam box, the second valve is proportionately opened thereby admitting air into the steam box.
Other objects and advantages of the present invention will be readily apparent to those skilled in the art by reference to the detailed description, drawings and the appended claims.